The invention relates to a method for producing methane stored in gas hydrates while at the same time storing carbon dioxide (CO2) in the geological subsoil.
Huge amounts of natural gas are stored in the sea bed as solid, ice-like methane hydrate. These natural methane hydrate deposits are therefore submerged and probably contain more energy and carbon (app. 3000 Gt C) than all conventional coal, oil and gas deposits on our planet. Therefore the gas hydrates play an important role as natural-gas source of the future. Methane hydrates were confirmed on almost all continental shelves at a depth in the water below approximately 400 m. They are stable only at high pressures and low temperatures. They occur where enough organic carbon was embedded into the sediment and the pressure and temperature conditions permit the fixation of methane in methane hydrates. Many coastal states have large national deposits (for example China, India, Japan, South Korea, Brazil, Chile, the US, Canada, Norway, Russia). On top of this methane hydrates were detected on land below thick permafrost deposits. These hydrate deposits are known above all from Siberia, Canada and Alaska.
FIG. 1 shows the phase diagram of methane in sea water. The methane hydrates are only stable at high pressures and low temperatures. The phase boundary between hydrate and gas applies for pure methane hydrate of the lattice type I and for sea water having a salt content of 35% by weight.
The phase boundaries applies for pure methane hydrate having the lattice type I. Methane hydrates exist in different lattice types. Type I is the most common and the most widely existing variant.
FIG. 2 shows a methane hydrate cluster of the lattice type I; in this type, there is on average one methane molecule for 5.7 water molecules. The methane molecules are represented by big spheres, while the small spheres connected by black lines represent the hydrate lattice that is made up of water molecules.
At the present time, methane hydrate deposits are developed all over the world to produce natural gas. To extract natural gas, the hydrates at first have to be broken down in the geological subsoil. In the process, the methane that is fixated in the water cages of the hydrates is released as gas that can be extracted by means of one or more boreholes using conventional technology. At present, it is essentially the following different approaches that are pursued:                the pressure in the deposit is lowered;        the temperature in the deposit is increased;        chemical substances are added to decompose the hydrates.        
U.S. Pat. No. 7,222,673 discloses to replace methane from the gas hydrates for carbon dioxide (CO2) without destroying the hydrate structure. In the process, the hydrates are brought into contact with liquid CO2. The reaction takes place without energy being supplied externally since the CO2 hydrates that are formed are more stable than the natural methane hydrates. This type of extracting natural gas has the added advantage that at the same time CO2 that is also responsible for heating up the Earth as a climate-relevant greenhouse gas can be safely stored underground and thus be removed from the atmosphere. A disadvantage of this method is the tow speed of the substitution reaction while maintaining the hydrate structure, which only allows very low production rates.
WO2005/076904 describes a method for storing CO2 under the sea bed by introducing gaseous CO2 into methane hydrate fields. CO2 hydrate is formed, the heat that is released leads to the dissociation of the methane hydrate and to the release of the methane. It is planned to collect and use the methane gas that has been released. The high content of gaseous CO2 is a disadvantage when using the methane gas that has been released for generating energy by combustion. The production rates that are possible are likewise low since the release of the methane from the methane hydrate is only possible with the aid of the heat that is released by the CO2 hydrate formation.